to they compromise it a lot? 'cause i wouldn't like to carry an aditional lens around... what kind of manufacters make good adapters and which one do i need for what kind of lenses? is there only one kind?

David, I think the experts here (not me ) could help you better if we knew what lenes you have, or what you want it to mount to, and what type of shots you would want to do.
I.E I have seen great shots of skateboarders, BMX, and street corners taken with fisheye's.
I have looked on ebay and have seen them from $59-$300(US)